Rare earth sintered magnets put into practical use are produced by pulverization of an alloy, molding, sintering, heat treatment and machining, and further surface treatment, if necessary. Among them, R-T-B-based rare earth sintered magnets having R.sub.2 T.sub.14 B intermetallic compounds, wherein R is at least one rare earth element including Y, and T is Fe or Fe and Co, as main phases are widely used as high-performance magnets. However, alloy powder is rapidly oxidized in the air, resulting in deterioration in magnetic properties. In extreme cases, rapid oxidation leads to ignition, posing safety problems.
Proposed as a method for preventing rapid oxidation are methods for producing a rare earth sintered magnet comprising introducing a starting material powder for the rare earth sintered magnet into a non-oxidizing mineral oil or synthetic oil, molding it in a magnetic field while preventing oxidation, and then carrying out oil removal, sintering and heat treatment in this order (see Japanese Patents 2,731,337 and 2,859,517). These methods provide sintered bodies having a low oxygen content and a high density almost equal to the theoretical density, which has remarkably improved maximum energy product (BH).sub.max.
Proposal was further made that remarkably improved continuous moldability is achieved by introducing the above fine alloy powder into an oil comprising a mineral oil, a synthetic oil or a vegetable oil and 0.01-0.5 weight % of oleic acid to form a starting material slurry for molding, thereby making it possible to efficiently produce a rare earth sintered magnet with improved magnetic properties (see Japanese Patent Laid-Open No. 8-130142).
However, the rare earth sintered magnets produced by the above methods have magnetic properties such as (BH).sub.max that are not so high as expected by the inventors, as shown in COMPARATIVE EXAMPLES described later, and further improvement in performance has been difficult. Also, when a thin (or thin and long), arc-segment-shaped green body for a rare earth sintered magnet is formed by compression molding in a magnetic field by the above conventional methods, remarkable cracking occurs. Further, the above thin (or thin and long) green body for a rare earth sintered magnet has an extremely uneven density distribution, resulting in largely deformed sintered bodies due to locally large differences in density. This leads to large deformation of the sintered bodies in an anisotropy-providing direction, resulting in decrease in orientation and thus failure to put them into practical use. Thus, these conventional methods fail to sufficiently satisfy the recent demand of making magnet products thinner, smaller and higher in performance. The term "thin" used herein means that the thickness of a magnet is as small as 4 mm or less, and the term "long" means that the axial length of a magnet is as large as 40 mm or more.
Japanese Patent Laid-Open No. 7-37716 discloses in EXAMPLE 2 that an alloy having a composition of Nd.sub.12.8 Fe.sub.bal. Co.sub.4.5 B.sub.6.2 Ga.sub.0.1 (at. %) is finely pulverized to an average particle size of 5 .mu.m, and that the resultant fine powder is mixed with an mineral oil and then subjected to molding in a transverse magnetic field under the conditions of an extremely high orientation magnetic field of 2.0 MA/m (25 kOe) and an extremely low molding pressure of 16.7 MPa (0.17 ton/cm.sup.2) without contact with the air, to provide an R-T-B-based sintered magnet with high magnetic properties of iHc=1.1 MA/m (14.1 kOe), (BH).sub.max =398.8 kJ/m.sup.3 (50.1 MGOe), orientation=96%, and I(105)/I(006)=1.32.
However, when thin (or thin and long) green bodies for arc-segment-shaped, R-T-B-based sintered magnets are formed by compression molding in a magnetic field under the conditions described in EXAMPLE 12 of Japanese Patent Laid-Open No. 7-37716, remarkable cracking occurs. Even when green bodies without cracking are obtained, they have an extremely uneven density distribution, resulting in largely deformed sintered bodies, leading to largely deformed sintered bodies poor in orientation, and cannot be put into practical use.
When radially anisotropic, R-T-B-based, sintered ring magnets (hereinafter referred to as radial rings) or arc segment magnets are formed under the conventional production conditions described in Japanese Patent 2,859,517, a radially orienting magnetic field should be applied from the inner surface side to the outer surface side of a cavity of a molding die in the course of molding to impart radial anisotropy to the green bodies, posing the problem that the smaller the inner diameter of a cavity, the weaker the radially orienting magnetic field. Thus, the smaller the inner diameters of radial rings, the poorer the radial orientation of green bodies. In actuality, if an orientation (static) magnetic field of more than 795.8 kA/m (10 kOe) can be applied in a radial direction for several seconds, it would be possible to obtain substantially the same level of radial orientation as the orientation of R-T-B-based sintered magnets formed though a molding step in a transverse magnetic field or a vertical magnetic field. However, in the industrial production of radial rings of 10-100 mm in inner diameter, the radially orienting magnetic field applied at the time of molding is as low as about 238.7-795.8 kA/m (3-10 kOe).
As shown in COMPARATIVE EXAMPLE 7 in Table 6, when a radial ring of 100 mm or less in inner diameter is produced using a starting material slurry described in Japanese Patent 2,859,517, high orientation cannot be achieved. As a result of investigation of these causes, the inventors have found that this is caused by poor radial orientation of green bodies.
Also, a radially orienting magnetic field applied during a molding step of radially anisotropic, R-T-B-based, sintered arc segment magnets in usual industrial production is as low as about 238.7-795.8 KA/m (3-10 kOe). Thus, like radial rings, the problem of poor radial orientation occurs in the case of R-T-B-based, sintered arc segment magnets of 100 mm or less in inner diameter.